The present invention concerns a liquid crystal display device and, in more detail, it relates to an electrically controlled birefringence (ECB) type liquid crystal display device in which molecules of liquid crystal having a positive dielectric anisotropy are aligned substantially homogeneously and light modulation is carried out by utilizing the variation of birefringence rate in accordance with an electric field.
A so-called ECB type liquid crystal display device utilizing the electrically controlled birefringence effect comprises a liquid crystal cell in which a liquid crystal layer is sandwiched between upper and lower substrates such that molecules of liquid crystal are aligned homogeneously (i.e., aligned in parallel to the surface of the substrate when a voltage is not applied), and a pair of polarizers disposed on both outer sides of the liquid crystal cell such that the transmission axis or the absorption axis of the polarizer adjacent to the liquid crystal layer is displaced from the aligning direction of the liquid crystal molecules, so that the liquid crystal cell is colored by utilizing the birefringence of the liquid crystal molecules. Then, the display color of the device is varied by the substantial retardation change of the liquid crystal layer in accordance with the change of aligning state of the liquid crystal molecules when a voltage is applied to the liquid crystal layer.
For utilizing the conventional ECB type liquid crystal display device as a light shutter, that is, as a black and white type light modulation device, it is necessary to reduce the product of the refractive index anisotropy .DELTA.n.sub.L and the thickness d.sub.L of the liquid crystal layer (.DELTA.n.sub.L .multidot.d.sub.L ; retardation of the liquid crystal layer) to about 0.3 .mu.m or less. However, since the change of the light transmittance in accordance with the applied voltage is remarkably decreased when the retardation of the liquid crystal layer is reduced, the device is not suitable for multiplexing driving and has a drawback that highly multiplexing driving thereof is substantially impossible.
On the other hand, in the STN type liquid crystal display device, there has been proposed a method of improving the black and white display of the device by further sandwiching, between the liquid crystal layer and the polarizer, a color-erasing liquid crystal cell in which liquid crystal molecules are twisted in the opposite direction (for instance, refer to H. Watanabe et. al; Society for Information Display 88, "Digest", pp:416-419 (1988), and Japanese Patent Laid Open Sho 64-519 (1989)). However, such a method has a drawback that the production steps for the device are complicated.
Further in the STN type liquid crystal display device, it has also been proposed to use a uniaxially stretched polymer film instead of the compensation cell as described above (for example, refer to H. Odai, et. al, 1988, "International Display Research Conference", pp:195-198). However, with such a method, the compensation is not complete and it is difficult to obtain an excellent black and white display device. Further Japanese Patent Laid Open Sho 62-210423 (1987) discloses use of a polymer film molded by a special method instead of the compensation cell in the ECB type liquid crystal display device of the DAP system. However, it is difficult by this method to produce films having homogeneous and predetermined birefringence.
The present inventors have made a study on the ECB type liquid crystal display device free from the foregoing drawbacks and, as a result, have found that black and white display having excellent multiplexing driving characteristic and excellent viewing angle property is obtained by sandwiching a birefringent layer having a predetermined retardation value between a liquid crystal layer and a polarizer such that the maximum refractive index direction within the plane of the birefringent layer is perpendicular to the aligning direction of the liquid crystal molecules in the liquid crystal layer. The present invention has been accomplished based on such a finding.
That is, it is an object of the present invention to provide an ECB type liquid crystal display device which does not have the foregoing drawbacks exhibited in the conventional black and white type ECB liquid crystal display device, is excellent in highly multiplexing driving characteristic, can be used suitably for a black and white light shutter or color display device and can be manufactured easily.